- Name: Sato Masaki (佐藤優樹)
- Nickname: Masaki (まさき), Maachan (まーちゃん), Masa (まさ), Masaking (まさキング)
- Birthdate: May 7, 1999
- Birthplace: Hokkaido, Japan
- Blood Type: A
- Height: 156cm
- Hobbies: Playing drums, riding horses
- Specialties: Piano, ballet
- Favorite Food: Soba, udon, plum sheets
- Favorite Color: Yellow, green
- Looks Up To: Niigaki Risa, Takahashi Ai
- Hello! Project groups:
The tenth generation made their official presentation on the same day they were announced as new members in the "Morning Musume Concert Tour Fall 2011 Ai Believe ~Takahashi Ai Graduation Memory Special~" concert, and they participated in a performance of the song "Tomo".
In June 2019 Masaki revealed that she began attending college sometime in April 2019, making her the first Morning Musume member to continue being active in the group while going to university.
On February 21, 2021 Sato was absent from an event due to severe stomach pain. The following day it was announced that Sato had been diagnosed with irritable bowel syndrome and would continue to be absent from performances while awaiting test results arriving the 26th. Announcements for her further break from activities involving performing or standing for long periods of time was made on April 27th, and a third update fans to her continued absence was given on June 3rd.
While still on break from performing due to her illness, Sato announced her graduation from Morning Musume and Hello! Project on September 24, 2021, to take place at Nippon Budokan during the "Morning Musume '21 Concert Teenage Solution ~Sato Masaki Sotsugyou Special~" on December 13th. A final single featuring Sato would also be released before the end of the year. The announcement stated that though she found some relief from her illness with a change of diet and medication she was still unable to participate in activities, especially performing, at a level to her satisfaction, and she felt it may have been causing trouble for the other members and staff. She chose to graduate to focus on improving her health first, instead of continuing in the group at a time when she could not reliably make plans for the future. She expressed her desire to continue a solo music career after her graduation in both the announcement and her personal blog.
- [2023.03.29] Ding Dong / Romantic Nante Gara ja nai (ロマンティックなんてガラじゃない)
- [2017.05.19] Miss Henkan!! (Miss変換!!) (Sato Masaki & Kudo Haruka)
- [2021.12.14] Egao no Kimi wa Taiyou sa (笑顔の君は太陽さ)
- [2012.07.11] Greeting ~Sato Masaki~ (Greeting ～佐藤優樹～)
- [2013.01.16] Alo Hello! Morning Musume 9 10ki DVD (Fukumura Mizuki, Ikuta Erina, Sayashi Riho, Suzuki Kanon, Iikubo Haruna, Ishida Ayumi, Sato Masaki, Kudo Haruka)
- [2022.02.28] prism
- [2012.09.10] Morning Musume｡ 9・10ki 1st official Photo Book (モーニング娘。9・10期 1st official Photo Book) (Fukumura Mizuki, Ikuta Erina, Sayashi Riho, Suzuki Kanon, Iikubo Haruna, Ishida Ayumi, Sato Masaki, Kudo Haruka)
- [2013.01.16] Alo Hello! Morning Musume 10ki Photobook (アロハロ！モーニング娘。10期メンバー写真集) (Iikubo Haruna, Ishida Ayumi, Sato Masaki, Kudo Haruka)
- [2013.09.30] Morning Musume 15th Anniversary Photobook ZERO (モーニング娘。 15th Anniversary Photobook ZERO) (Michishige Sayumi, Fukumura Mizuki, Ikuta Erina, Sayashi Riho, Suzuki Kanon, Iikubo Haruna, Ishida Ayumi, Sato Masaki, Kudo Haruka, Oda Sakura)
- [2014.05.30] Michishige Camera '13-'14 (ミチシゲカメラ'13-'14) (Michishige Sayumi, Fukumura Mizuki, Ikuta Erina, Sayashi Riho, Suzuki Kanon, Iikubo Haruna, Ishida Ayumi, Sato Masaki, Kudo Haruka, Oda Sakura)
- [2014.12.15] Alo Hello! Morning Musume '14 Photobook (アロハロ！ モーニング娘。'14写真集) (Michishige Sayumi, Fukumura Mizuki, Ikuta Erina, Sayashi Riho, Suzuki Kanon, Iikubo Haruna, Ishida Ayumi, Sato Masaki, Kudo Haruka, Oda Sakura)
- [2015.02.25] Michishige Camera 2~'14graduation~ (ミチシゲカメラ2~'14graduation~) (Michishige Sayumi, Fukumura Mizuki, Ikuta Erina, Sayashi Riho, Suzuki Kanon, Iikubo Haruna, Ishida Ayumi, Sato Masaki, Kudo Haruka, Oda Sakura)
- TV Programs
- [2011–2012] HELLOPRO! TIME (ハロプロ！ＴＩＭＥ)
- [2012–2013] Hello! SATOYAMA Life (ハロー！ＳＡＴＯＹＡＭＡライフ)
- [2014–2019] The Girls Live
- TV Dramas
-  Suugaku♥Joushi Gakuen (数学♥女子学園)
-  Ahare! Meisaku-kun (あはれ!名作くん)
-  KDDI au (as Morimusume)
-  Kagome (as Zenryoku Yasai Musume)
-  Marukome Morning Misoshiru
-  Nissay Mirai Sekapaca PLAY DREAM CAMPAIGN
-  AVIOT TE-D01q Wireless Earbuds
-  Michishige Sayumi no "Mobekimasutte Nani??" (道重さゆみの『モベキマスってなに？？』)
-  Guest, Maachan (ゲスト、まぁちゃん。)
- [2012–2013] Honki DE Hichou! (本気DE飛跳!) (fan club exclusive)
- [2013–] Hello! Station (ハロ！ステ)
-  Hello Pro no Oshigoto Challenge! (ハロプロのお仕事チャレンジ!)
-  Hello Pro Kouhaku Taiko THE☆BATTLE 2019 (ハロプロ紅白対抗 ザ☆バトル2019)
- [2019–2020] Hello Pro ONExONE (ハロプロ ONE×ONE)
-  Hello Pro no Oshigoto Challenge! 2 (ハロプロのお仕事チャレンジ!2)
-  Hello Pro no Collabo NIGHT! (ハロプロのコラボNIGHT!)
-  Stacy's Shoujo Saisatsu Kageki (ステーシーズ 少女再殺歌劇)
-  Idol Nihonryuu ~Onna Nichibu~ (アイドル日本流～おんな日舞～)
-  LILIUM -Lilium Shoujo Junketsu Kageki- (LILIUM －リリウム 少女純潔歌劇－)
-  TRIANGLE -Trine Angle- (TRIANGLE -トラインアングル-)
-  Zoku 11-nin Iru! Higashi no Chihei, Nishi no Towa (続・11人いる!東の地平・西の永遠)
-  Pharaoh no Haka (ファラオの墓)
-  Pharaoh no Haka ~Hebi Ou Senferu~ (ファラオの墓 ~蛇王・スネフェル~)
- [2012–] Morning Musume no Morning Jogakuin ~Houkago Meeting~ (モーニング娘。のモーニング女学院～放課後ミーティング～)
- [2017–] marukome Morning Misoshiru Nomou yo! (marukome モーニングみそ汁 飲もうよ!)
- Her official Morning Musume image color was emerald green.
- She is the sixth Morning Musume member to come from Hokkaido, after Ishiguro Aya, Iida Kaori, Abe Natsumi, Konno Asami, and Fujimoto Miki.
- Her favorite word is "deshou" (でしょう; "right?").
- Her motto is "Don't be noisy".
- Her philosophy is "being smiley".
- She believes she can't lose at having perfect pitch.
- She is left-handed.
- Has played the piano since she was 2 years old.
- Played the drums starting in her third year of elementary school, but stopped two years ago as of 2012.
- Is practiced to competition level obstacle jumping in horse back riding.
- Considers her charm point to be her eyelashes.
- Noted her favorite Morning Musume song to be "Maji Desu ka Ska!" in 2011, and "Fantasy ga Hajimaru" in 2012.
- Calls her father "Chichi" (父父) instead of "Papa" or "Otousan" (お父さん).
- Has two younger sisters.
- Has said that if she were not a member of Morning Musume, she would want to be a piano teacher.
- Spoke English until she was 5 years old, when she began kindergarten and had to start speaking Japanese. She grew up watching Disney Channel shows in English, and still does, despite no longer understanding the language.
- Notably called Tanaka Reina "Tanasatan" (たなさたん).
- Her favorite genre of music is J-Pop.